1. Field of the Invention
This invention relates to cardiac pacing systems and, more particularly, to dual chamber pacing systems designed to operate in a synchronous tracking mode but which control the circumstances under which a ventricular stimulus is synchronously delivered following a sensed atrial heartbeat.
2. Description of the Prior Art
The advantages of dual chamber pacing, and more particularly pacing in different modes which are selected in response to different patient conditions, is now well recognized in the art. Early pacing systems were solely ventricular, and were sufficient for management of patient with complete heart block and Stokes-Adams attacks. However, ventricular demand pacemakers are not equipped to take advantage of atrial activity, and thus are limited in their efficiency. Subsequently, atrial synchronous, ventricular pacemakers were introduced, having a lead for sensing P signals from the atrium and another for pacing the ventricle after a suitable P-R (A-V) interval. Such a pacemaker, e.g. VDI or VDD, allows the atrium to control the heart's response rate, the ventricle being paced at the atrial rate up to a predetermined upper rate limit. Such synchronous pacers have incorporated means for dealing with high atrial rates, including "block" and "Wenckebach" techniques.
Another form of A-V or dual chamber pacer that has been utilized is the sequential pacemaker (DVI), which paces both the atrium and the ventricle with an appropriate A-V delay which is timed by the pacemaker. A number of commercial pacemakers have been introduced which are programmable to these and other known pacing modes. Each of the various operating modes is particularly adapted to certain circumstances that may arise in a given patient.
Since the dual sense-dual pace DDD pacemaker became commercially available, it has gained favor for the reason that it compensates for many of the disadvantages of other pacemaker modes. The classic DDD pacemaker is described in U.S. Pat. No. 4,920,965, Funke et al., in some detail. See also U.S. Pat. Nos. 4,539,991 and 4,554,921, incorporated herein by reference, which disclose other forms of DDD-type pacemakers.
More recently, the DDDR pacemaker has come to prominence. In this type of pacemaker, there is provided one or more sensors which enable the pacemaker to be rate responsive, such that the pacing interval, or escape interval, is varied as a function of one or more sensed rate-indicating parameters, rather than being fixed at a programmed value. In the DDDR pacemaker, both atrial and ventricular natural beats may occur so long as they occur prior to the respective rate responsive escape interval. See U.S. Pat. Nos. 4,467,807 and 4,951,667, which are illustrative of dual chamber rate responsive pacemakers.
There have also been disclosed multi-mode pacemaker designs having means for switching modes in response to changing patient conditions. Most dual chamber pacemakers are programmable to distinct modes, or switch automatically from one mode to another under certain prescribed conditions. See, for example, U.S. Pat. No. 4,527,568, and U.S. Pat. No. 4,920,965. However, there remains a substantial need in the pacing art for sensing the conditions under which a dual chamber pacemaker can or should be controlled to change mode, and for providing optimum flexibility for blending two or more modes of operation. Thus, instead of forcing the pacer to operate in a distinct mode until patient history enables switching to another distinct mode, the pacer would optimally be enabled to react on a cycle-to-cycle basis to sensed events. For example, while it is desirable to synchronize a delivered ventricular stimulus to a sensed atrial signal whenever possible, at the same time the pacemaker should be controlled to adopt another more optimum response whenever desired. Thus, if an atrial sensed beat occurs at too high a rate, or if retrograde conduction is determined, the pacemaker should have maximum flexibility for responding to this situation.
An area of dual chamber pacemakers that has received a great deal of attention in the prior art is that of placing a high (upper) limit on the atrial rate that will be tracked, i.e., the rate at which a ventricular stimulus will be delivered at a synchronized A-V interval following the sensed atrial beat. As discussed above, the Wenckebach-type response is incorporated into many commercial dual chamber pacemakers, which provides for lengthening the AV interval in response to atrial beats above the ventricular upper rate limit. This response provides a degree of synchronization, and maintains the ventricular stimulus rate within than the upper rate limit. As is known, this results in the occasional loss of a synchronized ventricular stimulus. See U.S. Pat. No. 4,554,921, for an example of a pacemaker comprising both Wenckebach and block modes of operation. Another means of generating synchronous ventricular pulses in response to high rate atrial signals, whether or not premature atrial contractions, is to extend the post-ventricular atrial refractory (PVARP). Likewise, the atrial refractory time can be extended past the time of anticipated retrograde P waves so as to avoid pacemaker mediated tachycardia. However, the extension of PVARP or the atrial refractory, lowers the upper rate limit for sensing atrial signals, and thus reduces the capability of the pacemaker to provide synchronous operation in response to atrial heartbeats which may be physiological. It is also known to vary PVARP, or the upper atrial limit, as a function of sensor data; See also Funke, U.S. Pat. No. 4,920,965, where the atrial pacing timeout and effective PVARP are varied as a function of the timing of certain sensed atrial signals.
In view of the prior art, there remains a great need for a pacing system which substantially continuously senses the atrial signal, i.e., minimizes PVARP, and maximizes synchronous tracking of physiological high rate atrial signals while limiting tracking of non-physiological high rate atrial signals. It is important that the pacemaker track only physiological atrial signals, and avoid tracking non-physiological atrial signals. Further, there is a need for a system which not only can automatically go into asynchronous operation when nonphysiological atrial heartbeats are sensed, but can take active steps to re-synchronize the heart and return to synchronous operation as soon as possible.